1. Field of the Invention
This invention relates to a method of producing a self-supporting thin film of silicon single crystal of uniform thickness of about 0.5-10 .mu.m for use in the production of electronic devices, various types of sensors, micromachines and the like which utilize silicon semiconductors.
2. Description of the Prior Art
Silicon single crystal self-supporting thin films have conventionally been produced by the following methods.
1) First producing a silicon single crystal thin film of greater than final thickness and then mechanically and/or chemically polishing it to prescribed final thickness (Announcement of VIRGINIA SEMICONDUCTOR, INC., 1990).
2) Using an epitaxial growth technique to produce silicon having multiple layers with different impurity concentrations and then producing a thin self-supporting film by etching the silicon using an etchant whose etching speed varies with impurity concentration (1 I. Nashiyama, Phys. Rev. 19 (1979) 101, 2 S. Joksch, W. Graeff, P. Zaumseil, U. Winter, L. Csepregi, F. Iberl and A. K. Freund, J. Appl. Phys., 72 (1992) 54).
3) Thermally diffusing boron or the like from the surface of the silicon single crystal into the single crystal to thereby form a diffusion layer including the boron or the like as impurity and then removing the layers other than the diffusion layer by etching (N. W. Cheung, Rev. Sci. Instrum., 51 (1980) 1212).
Each of these methods has drawbacks, however.
Method 1)
Mechanical polishing can reduce the film thickness only to several tens of .mu.m and leaves a scarred surface. While the machining defects can be removed and the thickness be reduced by subsequent chemical polishing, the resulting thin film has very uneven thickness. Film thickness can also be reduced solely by chemical polishing but the resulting thin film is again uneven.
Method 2)
The complexity of the epitaxial growth process makes it difficult to control the film thickness.
Method 3)
Although the method employing thermal diffusion of boron is excellent for producing very thin films with a thickness of 1 .mu.m or less, the resulting film contains impurity (boron) at a high density of 10.sup.20 atoms/cm.sup.2. Moreover, since the thickness of the thin film obtained depends on the thickness of the diffusion layer, a thick diffusion layer has to be formed for obtaining a thin film but when this is done it becomes impossible to obtain a good quality single crystal because the number of crystalline lattice disorder is increased by dislocations and the like.
A strong need therefore exists for a silicon single crystal self-supporting thin film which has uniform thickness and enables easy control of film thickness and impurity concentration. The inventors accomplished the present invention through research aimed at meeting this need.